1. Field of the Invention
This invention relates to the disposal of animal waste, i.e., manure and is particularly concerned with a method of efficiently burning such waste material in a manner that is environmentally sound.
2. Prior Art
It has been estimated by the United States Department of Agriculture (USDA Report Misc. Pub. 1065, 1968) that nearly two billion tons of manure is generated annually in the United States. Using a density value of fifty pounds per cubic foot it has been calculated that this generated manure is enough to cover about thirty-five thousand square miles, i.e., the State of Indiana, with a one-inch layer annually. The problem is compounded by the fact that manure is produced in concentrated areas, such as feed lots, hog factories and large chicken factories, and in smaller animal processing operations, rather than being uniformly divided over the entire United States. Thus, large quantities need to be processed in local areas or both the large and small quantities need to be collected and transported to distant processing facilities.
Aside from the obvious odor problem associated with the processing of manure, other, not so obvious, problems exist. In many instances manure is mixed with water, as a result of the cleaning out of pens and stalls or by the falling rain and snow. The resulting contaminated water becomes a threat to streams, lakes and underground water supplies and ultimately to the drinking supply. Government agencies in areas of the United States having significant livestock operations are recognizing the dangers to the clean water supply and it has now become more difficult to obtain permits for large livestock operations in such areas. More recently it has become known that manure entering streams and lakes results in growth of organisms that attack and destroy fish in the streams and that even attack other animals and humans causing severe illness.
Even when used as fertilizer the animal wastes often present environmental problems that are costly and difficult to solve. For example, the manure generally contains weed seeds ingested by the animals with their feed grains. Present composting methods do not kill the weed seeds so herbicides are frequently added to the manure and when the manure is used as fertilizer the herbicide chemicals are added to the soil.
It is well recognized that when man creates environmental problems there is a cost associated with the clean-up or avoidance of the problem in the future. Trash dumped in the oceans, manufacturing process pollutants discharged into streams, rivers and lakes, exhaust emissions from automobiles and nuclear wastes are examples of environmental problems that are currently being addressed at great expense to the American taxpayer. Animal wastes represent just one more environmental problem that must be addressed to insure quality life for humans. Clearly, there is a need for methods and systems to dispose of manure, on-site, in a neat, cost effective manner.
At the present time current approaches to animal waste management are as old as the problem itself. Often it is merely spread on the ground as fertilizer or compost. Other times it is dumped into lagoons. Manure spread on the ground or placed in piles or in lagoons not only takes up large amounts of valuable ground space but creates incredible odors. The odors have resulted in the treating of the manure with chemicals to reduce or change the nature of the odors. The use of chemicals results in increased cost in the processing of the manure and the chemicals may not always be environmentally safe.
In most situations manure represents an expense and pollution liability rather than a marketable fertilizer product. In some instances, chicken litter (excreta and bedding material) can be used for cattle feed. In a limited number of areas manure is sold or given away. For most animal operations the manure is simply a nuisance. For producers unable to simply pile up manure there are, at the least, handling and transportation costs involved in moving the manure to a disposal location. Typically, for hogs, manure is produced approximately at the rate of two to three pounds per pound of weight gain. A hog will produce about six hundred pounds of manure over its four and one-half to six month life span. A producer marketing one thousand head per year would have about three hundred tons of raw manure to deal with. Since hog wastes are typically washed out of the hog pens, the total weight to be handled is probably three to four times the raw manure weight, or about one thousand tons. In addition to the costs involved in handling such large quantities of waste, it is noted that there are fewer and fewer locations where sites for the dumping of the large amount of waste can be handled. Consequently, the manure is confined to lagoons and becomes a nuisance to the producer and his neighbors.
There can be no doubt that there is a need for a method and system to dispose of manure, on-site, in a neat and cost effective manner.
1. Brief Description of the Invention
The present invention provides a method and system to combust manure. More particularly, using coal as fuel for the method of combustion.
2. Objects of the Invention
Principal objects are to provide a method and system for the burning of manure in an economical manner suitable for use by at least medium and large sized size animal operations.
Other objects are to provide for the processing of animal wastes to eliminate the health, safety and nuisance aspects while utilizing the inherent virtues of the material, i.e., its water content and its value as an energy source and as fertilizer. The heat generated by burning the animal waste, in conjunction with coal, can be readily used to create heated air, hot water or steam suitable for heating farm buildings. The animal waste can be collected and stored for such short periods of time as may be necessary to concentrate burning operations according to when the generated heat can be efficiently used for building heating. For larger animal raising operations the heat generated from the burning of the animal waste can efficiently be used in the generation of electricity to be sold or used in the farm operations.
Since most animal wastes contain a significant amount of moisture it is an object of the invention to provide a method and system for burning animal wastes containing high water content, (up to about 75%) and including slurry wastes that have been stored in lagoons or ponds.
3. Features of the Invention
In practicing the method of the invention animal waste is subjected to primary heat exchange to evaporate the bulk of moisture from the waste. The dried waste is then mixed with a combustible supplemental fuel and is burned. Water vapor and gasses separated during the primary heat exchange are processed to recover the water, the gasses and much of the heat used in the primary heat exchange. The recovered gasses are burned with the dried waste and supplemental fuel. Dry ash resulting from the burning of the dried waste, supplemental fuel and recovered gasses is collected for use in producing fertilizer. Exhaust from the burning of the dried waste and supplemental fuel is stripped to provide a clean exhaust suitable for discharge to atmosphere. Fly ash separated from the exhaust is mixed with dry ash in producing fertilizer. Excess heat, i.e. the heat in excess of that amount needed for drying, produced from the burning of the dried waste and supplemental fuel, is discharged to a use location, which may be a boiler, furnace, another system, or a building heating system.
The system used to perform the method of the invention includes a primary heat exchanger that will dry wastes without discharge of repugnant odors to the atmosphere. Preferably, the primary heat exchanger is heated using the heat generated by burning of the dried waste and a supplemental fuel. The primary heat exchanger may be variably constructed, depending upon the specific characteristics of the waste material and appropriate engineering considerations. However, apparatus providing for the isolation of evaporated gasses, the condensation of water vapor, and the subsequent burning of the non-condensable gasses is necessary to practice the method.
The presently preferred primary heat exchanger/dryer includes a stacked set of belt conveyors mounted within a housing. Heated air is passed through double-walled enclosure passages, around the belt conveyors transporting waste material and through horizontal passages formed in the heat exchanger/dryer. The heating of the waste material evaporates water and releases other gasses from the waste material.
A cyclone assembly, including a blower, separator, and condenser pulls primary evaporation products out of the primary heat exchanger/dryer, thereby producing a drying of the waste material. The belt conveyors transport and agitate the waste material to maximize exposure of waste material surface and to increase evaporation from the waste material. Preferably, also, the belts have cleats attached thereto to break up the waste material placed on the belts and brushes are used to prevent caking or buildup of waste material on the belts.
A first, or top endless belt, conveys waste material placed on the carrying surface thereof in a forward direction to the discharge end of the conveyor and dumps the material onto a heated skid plate. Brushes, serving as spaced cleats projecting from the carrying surface of the conveyor belt, sweep the heated skid plate during the return run of the endless belt. The brushes clean the heated skid plate and sweep the waste material dumped onto the heated skid plate, with a rolling motion, to and over an end of the heated skid plate where it drops onto the next or second belt. The waste material is transported by each endless belt and heated skid plate set, in the manner previously described, until it is thoroughly dried and is dropped onto a dried material conveyor and from that conveyor is dropped into a dry manure hopper that feeds the dried waste material onto a feed conveyor moving a mixture of waste material and coal into a burner assembly, where it is burned. The number of endless belt and heated skid plate sets provided depends upon factors such as the initial water content of the raw waste material, the waste volumetric rate, and the amount of heat that is allocated for drying versus use for other purposes. Drying can be accomplished at even moderate temperatures since a cyclone separator will maintain a low dew point by extracting vapor as it evaporates, thereby carrying the vapors away before they can recondense. A low air flow velocity within the conveyor prevents dust from being carried out with the products of evaporation.
A cyclone separator draws exhaust gasses from the primary heat exchanger at below atmospheric pressure to separate water from the exhaust and returns the non-condensable gasses, including to a large extent, those odiferous gases that are water soluble and that might otherwise recombine with the separated water, to the retort of the burner assembly. Thus, the noxious odors are maintained isolated and do not escape to atmosphere.
The cyclone separator includes a heat exchanger section in which condensable gasses, primarily water vapor, will condense. In condensing, the water vapor will give up heat of vaporization, allowing a portion of the heat used for evaporation to be recovered. Water leaving the cyclone condenser is filtered in conventional fashion to remove water soluble materials, if any. Heat recovered from the cool side of the condenser heat exchanger is convected away with water or air flow and may be used in preheating functions, such as preheating of the raw waste material.
Gasses exhausted from the cyclone separator are piped to the inlet of the combustion air blower. Fresh air may also be added to the inlet of the combustion air blower. The amount of fresh air added is determined by the oxygen content of gasses coming from the cyclone separator, which oxygen content may be determined experimentally or by measurement. The fresh air and gasses from the cyclone separator are then forced into the burner assembly to supply oxygen for the complete combustion of the dried waste material and supplemental fuel. All gasses associated with the waste material and drying of the waste material are maintained separated from atmosphere throughout the process and any heat value from hydrocarbons is liberated in the burning process.
Preferably, the supplemental fuel used to burn the animal waste is crushed coal that is mixed with the dry animal waste in a stoker system having two fuel hoppers on a single screw conveyor feed line. The proportions of animal waste fuel and coal fuel are readily adjusted as necessary to produce desired combustion characteristics. Such characteristics include total heat release, gas temperature, and percentages of carbon dioxide, carbon monoxide, and oxygen. As in all combustion systems, the general goal is to maximize carbon dioxide and minimize oxygen and carbon monoxide. The additional product of combustion, water vapor, to a great extent, will be trapped in the combustion gas cyclone.
The preferred burner into which the combined fuels are transported is an underfeed coal stoker retort and tuyere assembly. The screw conveyor augers the fuel mixture upward from the bottom of a retort bowl into the burning zone. The tuyeres direct combustion air into the burning zone. As the fuel is consumed the remaining ash is pushed radially away from the burning zone onto an ash removal ring. The ash removal ring, rotating at a slow rate and having a fins on an upper surface, moves ash circumferentially until it drops into a trough containing an ash removal auger. Ash is then transported to a collection receptacle for subsequent use as fertilizer. The under feed stoker works well for coal firing rates up to 1500 lbs/hr. For larger applications other coal burning systems, such as those used for large industrial plants and in power generation plants, may be used.
While crushed coal is the preferred supplemental fuel, other fuels can be used in practicing the method of the invention. Typical suitable fuels include pulverized coal, saw dust, wood chips, chicken litter, nut shells, and other bio-mass materials with sufficient energy content to support a clean burning, high temperature combustion process. To accommodate the various fuels, a curved diverter or baffle is provided within the combustion chamber to re-circulate light particles which may be blown out of the primary combustion zone. Unburned particles entrained with the combustion gasses are directed back down into a secondary combustion zone, along with combustion air injected through the diverter, to maximize combustion efficiency. Combustion gasses then travel laterally along the diverter in order to reach the flue. Recirculation and forcing of the gasses around edges of the diverter before exiting also aid in reducing the amount of fly ash carried from the combustion chamber with the gasses.
Combustion gasses exit the primary fire box and pass into a secondary fire box section. The secondary section provides additional surface area for heat exchange with circulation air. The secondary fire box section also serves as a trap for fly ash. Combustion gasses are forced to travel down past a deflector partially covering the flue gas outlet. The rapid change in gas direction during the exhaust process helps separate particulate material from the gas stream. The fly ash so separated settles to the bottom of the secondary section and passes through an opening in the fire box floor. The opening connects into the ash removal trough and the fly ash is conveyed out of the furnace along with the ash separated in the fire.
Additional objects and features of the invention will become apparent from the following detailed description and drawings.